The practice of baker's yeast production is well known and amply documented in the literature. Good examples of descriptions of the practice of baker's yeast production are e.g. Burrows, S. (1979) Baker's yeast, Economic microbiology, vol. 4 (Rose, A. H. ed.), pp. 31-64; Academic Press, New York; Reed G. (1982) Production of bakers' yeast, Prescott & Dunn's industrial microbiology, 4th ed. (Reed, G. ed.) pp. 593-633; AVI, Westport, Conn.; Chen, S. L. and Chiger, M. (1985) Production of baker's yeast, Comprehensive biotechnology, vol. 3 (Blanch, H. W., Drew, S. and Wang, D. I. C eds.) pp. 429-461; Pergamon Press, Oxford; Trivedi, N. B., Jacobson, G. K. and Tesch, W. (1986) Baker's yeast. Crit. Rev. Biotechnol. 4, 75-110; and Beudeker, R. F., Dam, H. W. van, Plaat, J. H. van der, and Vellenga, K. (1990) Developments in Baker's yeast production, Yeast (Verachtert, H and De Mot, R. eds.) pp. 103-146; Marcel Dekker Inc., New York. Below, further attention will be given to specific aspects of the production and use of baker's yeast that can be improved with the invention described in this specification.
After the production of the seed yeast in multiple stages (Chen, S. L. and Chiger, M. (1985) Production of baker's yeast, Comprehensive biotechnology, vol. 3 (Blanch, H. W., Drew, S. and Wang, D. I. C eds.) pp. 429-461; Pergamon Press, Oxford) the production of the so-called commercial yeast follows. In standard practice this is done in fed-batch fermentations mainly using molasses as the C-substrate and ammonia or urea as the main nitrogen source. The substrates are fed to the fermenter during the fermentation. Other growth requirements like phosphate, part of the nitrogen, salts and vitamins are added to the fermenter at the start of the fermentation or in the very first hours of fermentation. Molasses functions also as the source of many trace elements, that are dosed in sufficient or even excessive amounts by feeding the molasses as C-source. The fermentation takes between 10 and 20 hours and ends with a broth containing between 4 and 8% dry yeast solids.
Before it can be used, the molasses needs to be clarified. This means that the molasses is diluted in order to lower the viscosity and make the molasses pumpable, but also to allow removal of sediment (sand, dirt, colloidal matter) before sterilization and feeding to the fermenter.
The feed schedules used for the molasses and nitrogen source and to some extent also the other growth requirements are generally considered as critical knowledge and not much is published about the schedules actually used in industrial practice. It is clear however that the schedules are of prime importance for the final yeast quality obtained. As is clear from Burrows, S. (1979) Baker's yeast, Economic microbiology, vol. 4 (Rose, A. H. ed.), pp. 31-64; Academic Press, New York, and earlier work of Drew, B. von, Specht, H. and Herbst, A. -M. (1962) Zur Zuchtung von Backhefe in konzentrierter Melassewurze. Die Branntweinwirtschaft 102, 245-247, higher molasses feed profiles lead to more active yeast and lower molasses feed profiles to less active yeast having a longer shelf-life. In current practice, the maximal feed rate is limited on the one hand by the oxygen transfer rate (OTR) of the fermenter and on the other hand by the critical growth rate of the yeast above which formation of alcohol starts. Formation of alcohol is undesirable because of the resulting poor keeping quality of the yeast and loss of yield on carbon source. Apparently a too low molasses feed profile relative to the amount of yeast in the fermenter can lead to too low gassing activity of the yeast. Thus in Sher, H. N. (1962) Continuous process for the production of yeast, U.S. Pat. No. 3,032,476, it is stated that the growth rate of the yeast should be maintained above 0.05 h.sup.-1 and preferably even above 0.075 h.sup.-1. So, given an economically relevant inoculum percentage, the minimum growth rate considered necessary for good gassing performance together with the maximum feed rate due to fermenter oxygen transfer rate limitations forms the basis for the maximum fermentation time of 20 h as stated by Chen, S. L. and Chiger, M. (1985) Production of baker's yeast, Comprehensive biotechnology, vol. 3 (Blanch, H. W., Drew, S. and Wang, D. I. C eds.) pp. 429-461; Pergamon Press, Oxford.
After the fermentation the yeast cells are washed thoroughly by repeated concentration and dilution. Typically a centrifugally concentration is done to a suspension of about 20% dry solids and the suspension is at least once diluted to more than 100% of the original volume, resulting in a non-yeast solid concentration in the free liquid of less than 10% of the concentration in the free liquid phase of the fermentation broth. Thus, a cream yeast is obtained with a yeast dry solids concentration of 18-22% which is either sold directly as cream yeast or further processed into block-yeast or granulated yeast (25-36% dry solids) or dried to obtain active dry yeast or instant dry yeast with up to 97% yeast dry solids. The extracellular water removed from the broth in this way amounts to about 50% for cream yeast, up to almost 100% for the dried yeast. Together with the water required to wash away non-fermented solids from the molasses, this water forms a large stream of waste-water that needs to be handled. Nowadays full waste-water treatment includes an evaporation plant concentrating the waste-water stream and yielding vinasse. At the expense of a high energy input, this step removes about 80-95% of the biological oxygen demand (BOD) from the waste-water stream. The remaining BOD is treated in an anaerobic waste-water treatment plant and subsequently in an aerobic waste-water treatment plant, again at considerable cost. Moreover, these costs will increase in the future as energy becomes more expensive and also demands for treatment of waste streams increase for environmental reasons.